Sound translating device



July V26, 1932.

A. L. THURAS SOUND TRANSLATING DEVICE Fil'ed Aug. 15, 19:50 3sheets-Sheet 1 FIG.

70 80 90 |00 FREQUENCY-N PER SECOND INVENTOR A. L; THURAS A TTORNEV July2 6, 1932. A. L. THURAS SOUND TRANSLATING DEVICE 3 Sheets-Sheet 2 FiledAug. 15, 1950 Y F/G. 6

/NvE/vron A. THURAS Arrow/vir- .muy4 26, 1932.

A. THuRAs 1,869,178

SOUND TRANSLATING DEVICE 3 Sheets-Sheet 3 Filed Aug. l5, 1950 0mm MATTORNEY Patented July 26, 1932 UNITED STATES PATENT NoFl-lcE ALBERT L.Taurus, or' NEW Yoan, N. Y.,

i TORIES, INCOIREORAZI.El),.OFr NEW YORK, N. Y., A. CORPORATION OF NEWYORK 'soUNnrnANsLATING nEvIcE Application ledAugust 15, 1930. Serial No.475,468.

This invention relates to sound translating devices and moreparticularly to a method and means for increasing the response of suchdevicesi at the lower frequencies.

translated and in a more proportionate dei0 gree to the' higher notesthan hasv been possible hitherto.

One feature of this invention comprises utilizing the 'sound vibrationstranslated by the two surfaces of a diaphragm to eEectively'reenforceeach other by causing the sounds translated by one surface to.be changedin magnitude and phase with reference to those translated by the othersurface.

Another feature of this invention resides in its application to a loudspeaking receiver whereby the radiation resistance per unit area of thediaphragm thereof at the lower frequencies is effectively increased byproviding an acoustic path from the rear surface to the front surface ofsaid diaphragm which at its outlet, in close proximity to saiddiaphragm, is substantially the same area as that of the diaphragm, saidacoustic path defining an acoustic lter.

Another feature of this invention Iis its application to a device fortranslating acoustic vibrations into electrical vibrations whichconsists in increasing the amplitude of vibration of the vibratoryelement of said device by causing a portion of theacoustic vibrations tobe translated to be directed against the rear surface thereof afterhaving passed through an acoustic filter so proportioned as to stiffnessand mass that such acoustic vibrations act to reenforce the effect ofthe vibrations impressed on the front surface of' mass element,respectively, of an acoustic filter, so proportioned that when thediaphragm'is actuated to translate acoustic vibrations, the vibrationsparticularl those of the lower frequencies", translated y each surfaceof said diaphragm, are caused to reenforce each other to increase theresponse of said sound translating device.

A more complete understanding of this invention will be obtained byreference to the appended drawings wherein:

Fig. 1 is a front view of a sound translating device embodying featuresof this invention;

Fig. 2 is a sectional view taken along the line 2-2 of the device 'shownin Fig. 1;

Fig. 3 shows a simplified schematic sketch of the device shown-in Fi 1and 2;

Fig. 4 illustrates the equivalent impedance circuit of the device shownin Figs. 1 and 2; Fig. 5 shows graphically the operating.characteristics of a" sound translating device such as shown in Fig. 1;

Fig. 6 shows a view partly in section of another embodiment of thisinvention;

Fig. 7 shows a front view of still another embodiment of this invention;

Fig. 8- is a sectional view of the embodiment shown in Fig. 7 takenalong the line 8-8; 4.

Fig. 9 shows al sectional view of a telephone transmitter incorporatingfeatures of this invention;

Fig. 10 is an electric circuit analogy of the acoustic elements involvedin the operation of the device shown in Fig. 9;

Fig. 11 is an electric circuit analogy of the acoustic elements involvedin the operation of a device similar to that shown in Fig. 9 withoutfeatures of this invention;

Fig. 12 shows graphically the operating characteristics in the lowportion of the frequency range of the device shown in Fig. 9 and of thesame device without features of this invention; and

Fig. 13 shows typical overall response characteristics of the deviceshown on Fig. 9 vincorporating and not incorporating the features ofthis' invention.- V

Referring now to the drawings, there is AssIeNoB 'ro BELL TELEPHONEmomi- Vso showin in Figs. 1 and 2, a sound translating I device of theloud speaking receiver type embodying the features of this inventionconsisting of enclosure member l wh1ch 1s here shown as of rectangularand box-like shape whose wall elements are held together by'screws 3 andwhich provides a fluid chamber 5 behind a vibratory element or diaphragm7.

The diaphragm 7 may be of a l1ght material such las an alloy of aluminumor other suitable material. It comprises a curved central portion with asurrounding reversely curved portion terminating in a flat rmi. Thediaphragm isV shown supported 1n an aperture in the front wall 15 of theenclosure member 1 and at its periphery 1s secured to said wall betweenmounting members 8 in any suitable manner, for example, by screws 14.Current coil 9 is attached to the inner or rear surface of the diaphragmand is dis osed in the magnetic air gap 16 provided y the magnet 10which may be either of the permanent magnet or of the electromagnetictype. Central pole portion 17 of the magnet is preferably hollow therebyproviding a passage-way y11 whereby sound disturbances radiated by therear surface portion of the diaphragm inside the driving coil 9 mayreadily pass into the chamber 5. The magnet 10 is supported on the crosspiece 13 which is, in turn, attached to a plurality ofvertical supports12 secured to the inner surface of the front wall 15.

passage from the chamber 5 to the atmosphere adjacent the front surfaceof the diaphragm. These tubular members are pref-4 erably situated asclosely as possible adjacent the periphery of the diaphragm. As

shown, each tubular member is at one extremity substantially Hush withthe front wall 15 of the enclosure member and has its other extremityprojecting into the chamber 5. When the diaphragm 7 is caused to vibrateby the movements of coil 9 in response to the passage therethrou h ofcurrents which produce a variable geld interacting with the fixedstrength field of the magnet 10, both the front and the rear surfaces ofthe diaphragm will cause acoustic disturbances to be propagated throughthe fluid thereadjacent. The sound waves generated by the rear surfaceof the diaphragm, particularly the low frequencies, are caused to beconducted through said chamber and outlet passal e to combine with thesound waves generate b the front surface of the diaphra m' an lowrequencies.

In order that this invention maybe more readily understood it isbelieved that a dis- 'cussion ofthe theoretical considerations involvedin its operation and application to the loud speaker receiver of Figs. 1and 2 should be outlined.

Theory and measurements show that generally speaking, the directradiating type of loud speaker of moderate size, has a relatively smallresponse and inadequate power output capacity at low frequencies. By themethod comprising one feature of this invention and with a structure asdescribed above for increasing the response of loud speakers at lowfrequencies, both sides of the diaphragm 7 are utilized and theradiating resistance per unit area of the diaphragm is therebyincreased. By means of an acoustic stiffness and mass, the airdisplacement from the back of the diaphragm is shifted approximately 180in phase and led out at the front of the diaphragm. The acousticstiffness is obtained from the volume of airin the chamber 5 at the backof the diaphragm and the acoustic mass from the air in the outletpassage comprising the paths 6 in the tubular members 4 leading from thechamber to the front of the diaphragm. The phase relation of the volumevelocity of the diaphragm and the air to reenforce the same at the inthe passage opening is somewhat similar to that in an electricallow-pass filter in which the currents in the two adjoining series arms(containing small resistances) are nearly 180 out of phase above thecutoff frequencyV of the filter. rIhe acoustic system as regards Vpoweroutput differs considerably from the electrical filter in that theradiation resistance and air mass of the diaphragm and the correspondingradiation resistance and air mass of the passage have a mutual reactionon each other and furthermore their acoustic impedances vary withfrequency. A solution of this problem is rather complicated, but bymaking a few simplifying assumptions a general idea of the gain in soundpower output can be obtained. Ifthe area of the passage is taken equalto the effective area of the diaphragm and a frequency such that thevibratory velocity of the diaphragm is equal to the vibratory velocityof the air in the passage, then the effective radiating area 1s doubledand the radiation resistance per' Passage ingly over that which wouldobtain at ,the same frequency with the passage closed. This will give anincreasingly higher gain down to the v cutoi of the acoustic system.Above the fre-4 jphase shifting acoustic system has been Worked out andapproximate equations -formulated for the case in which the area of thepassage is equal to the effective area of the diaphragm and the mass ofthe mechanical moving element (diaphragm and coil) is equal to the massofair in the passage.

The response of a loud speaker connected to the output of a vacuum tubemay be expressed in terms of the acoustic power radiated when a constantvoltage is impressed in series with the electrical impedance of thedriving coil and the impedance of the source. The acoustic power isobtained by (l) deriving an expression for the velocity of the diaphragmand of the air in the passage in terms of the acoustic impedance andconstant impressed voltage in the electrical circuit; (2) deriving anexpression for the acoustic impedance of the air and the mutualimpedance caused by the velocity of the diaphragm and velocity of theair in the opening. The

` power output is then the sum of the products of the square of thesevelocities and corresponding resistances.

Refer now to Figures Sand 4 which show a simplified schematic sketch ofthe loud larger resistance load on the diaphragm sidered` are longcompared with the dimenptxiis of the acoustical system.

E=constaut voltage in the electrical circuit.

ma' F=mcchanical iorce-----IOR VZ=impedance in the mechanical circuitcaused by the electrical circuit.

which is equal to Z =II%0:-If the impedance iu the electrical circuit isassumed to be pure resistance equal to R.

H==e1d intensity l=length of wire in the moving coil` J.'lfcdgestiffness rcactance of the diaphragm S j-u stiness reactancc of volumeof air in the enclosure.

jwm=mass roactance of the diaphragmL-:mass reactance of the air in theopening.

V,V|, Vz=velocities in the various parts of tne acousticalmechanicalsystem.

V 2V AZ and -Vz AZ=air impedances imposed on the diaphragm and onopening respectively.

A=eective area of the diaphragm=areaof the outlet passage.

Z=ar impedance per unit area ot an area twice the area of the diaphragm.

In the impedances and W2 Kirchhoiis laws to the circuit in Fig. 4:

speaker7 and its equivalent impedance circuit. F: V1 V1 jwm V1 +A .ZV+ VFig. 3 shows a single opening 18 only lead .70 .Tw ing from the chamberbehind the diaphra AZ V S i but the theoretical considerations are t e0=3mv2+ 2 +52, V same regardless of whether one or more open- V: V .Vings are employed if the area relations here 1 2 considered are notdeparted from. If Z=T+Jx To simplify the calculations the followingassumptions have been made-(1) the area Then the aCOUSlC pOWer Output 1sI real V 2 real V i? Ar 2,

www 2V1AZ i V1 Lpm 2V2AZ)V2 2 (l) of the outlet assage from the .chamberis equal to the e ective area of the diaphragm5 the .eii'ective mass ofthe moving element, and (3) the wavelengths of the frequencies con- V:S., 2. 5' Z.,AZ Q AZS0 S S .2

-1(2) the mass4 of air in the opening is equal to Substituting inequation (1), expressing F in terms of E and dividing by 10', gives theacoustic power output in watts.

.low frequencies.

I- Tlie following'electrical,*mechanical and acoustical constants wereused in an experiimentalloud speakerincorporating the features of thisinvention:-

Z -pCS(X IY) where p=density of uuid C =velocity ot sound S=area ofdiaphragm considered as pstoa X== resistance factor of diaphragmconsidered as piston Y=reactance factor of diaphragm considered as apiston.

spaced 3 in. holes were provided in which were fitted tubular members ofphenol fibre 6 in. in length. These tubes were used because they werefoundeasier to construct than the single opening for the loud speakerbox. In this particular embodiment ofthe lnvention, the volume of theair in the chamber and in the tubular members 4 gave an acousticstiffness and mass respectively which had a cutoztl:l frequency of 40cycles. v l

The full line and the dotted line curves in Fig. 5 picture the soundoutput to be eX- pected when aloud speaking device, such as describedabove, is operated with passageways connecting front and rear surfacesof the diaphragm closed and opened respectively, the source of impedancebeing equal to twice the impedance of the coil attached t0 the diaphragmof the speaker. These curves were plotted from power output calculationsbased on Formula `(2) above. The broken line curve is drawn throughvalues of response at particular frequencies obtained by taking thedifference between measured values of response for the loud speaker withthe passage-ways opened and closed and using the calculated response forthe loud speaker with the chamber outlet passage-way closed as a baseline. It will be apparent from these curves that the actual response ofaloud speaker embodying the features of this invention agrees closelywith the theoretical response to be expected. If it is desired to avoidany pronounced peak in the response near the cutoff acoustic dampingmaterial may be placed in the outlet passage.

It is apparent that this invention not only increases the relativeresponse, but also increases the sound power output capacity 'of directacting types of loud speakers in the The air displacement from aloudspeaker with this acoustic system is and the ratio of the airdisplacement to the diaphragm displacement increases as the freh quencydecreases thus compensating to a large extent for the decrease 1nradlatlon resistance Since direct acting loud speakers are limited intheir power output calpacity by the amplitude of motion of the diap ragmat'lowfrequencies, the importance of this acoustic system is evidentwhen the large acoustic powers developed by the long pipes ofthe organand Ithe low frequency instru- Iments of the orchestra are considered.This lgreater than the 'diaphragm displacement if' invention will permitthe reproduction of the low frequencies of speech and vmusic in moreproportionate degree to the high frequencies than has been hithertopossible.

It is to be understood that the descri tion of the experimental loudspeaking device uilt in accordance with this invention is of anillustrative nature only and is not intended as a limitation on thescope of this invention nor is it .intended that the recitation of whathas been found to be a desirable relation between the diaphragm area andthat of an outlet passage connecting the Huid adjacent each surface ofthe dia hragm and that between vthe mass of the iuld in said passage andthat of the diaphragm and coil should be construed as a limitation. Itwill be apparent to those skilled in the art that variations instructure and proportions may be made without departing from the essenceof the invention.

Fig. 6 shows al side view partly in section of. another embodiment ofthis invention. Instead of a rectangular enclosure behind the rearsurface of the diaphragm, a cylindrical member 22, which may be ofmetal, is at tached to an annular member 24 which supports a diaphragmv7 similar to that of Fig.

2, and a plurality of tubular members 4 pro- /viding an outlet passagefrom within the chamber 28, provided by said member 22 and its curvedbase 27, in a manner similar to that described with reference to Figs. 1and 2. Magnet 10 is supported by a cross piece 19 which, at itsextremities, is attached to angle pieces 29 in turn riveted or otherwisesecured to the inner wall of the member 22. The base 27 of the member 22is preferably slightly curved to avoid the possibility of a drum headeffect when the loud speaker is in operation and the member 22 ispreferably cylindrical `in order'to substantially eliminate the eiectwhich is provided also with a. hollow central pole as shown in Fig. 2.Cylindrical tubes 33 and 34 held in spaced relation by the separatingmembers `38 provide an annular passage vconnecting the chamber 32 withthe atmosphere adjacent the front surface of the diaphragm 7. The magnet10 is supported b the cross piece 42 which at its extremities 1sattached to angle pieces 43, which are in 'turn secured to the innersurface of the tube 33. The diaphragm, its actuating mechanism and thespaced tubes 33 and 34 may all be supported by the front wall 30 of thechamber 32,

as shown, being secured thereto by screws 37.Y

The air in the chamber 32 and that enclosed between the tubes 33 and 34have such stiffness and mass respectivel that they form elements of anacoustic lter whereby the sound vibrations translated by each surface ofthe diaphragm at the low frequencies act to reenforce each other,thereby `increasing the relative response and power output of the devicein the low frequenc region.

This invention is not limited in its application to sound translatingdevices for reproducing sound, but may be applied to sound translatingdevices for changing acoustic vibrations into electrical vibrations. InFig. 9 there is shown a transmitter ofthe moving coil type comprising' ama et structure which is preferably of t e ermanent magnet type having acentral po e portion 62 to which a curved pole piece 51 is secured bymeans of the screw 63. The other pole piece consists of an annular platemember 56 secured t the magnet by screws 57 and having a centralaperture, the bounding wall of which is in spaced relation to the polepiece 5l thereby providing a magnetic air i gap in which a current coil61 attached to the diaphragm is disposed. The diaphragm 60 has a curvedcentral portion surrounded by a flat portion and is preferably of alight metal, suchk as alloy of aluminum, although any other suitablematerial may be used therefore. Surrounding the upper portion of thecentral pole and the magnet immedi- 'ately under the pole piece 51 is anannular member 52 which provides a very shallow recess 65 under themoving coil to insure suiiicient clearance for. the coil when vibratingand a narrow passage-way 66 therefrom leading into the hollowed portionor chamber 53 of the magnet. The diaphragm 60 is supported at itsperiphery and clamped between ring members 55 and 59 by screws 58 whichthread into plate member 56. The ring 55 is provided with a taperedinner surface 67 to substantially eliminate the increase in pressure onthe diaphragm caused by the resonance of the chamber above thediaphragm. Extending through an opening provided in the ring members 55and 59, the diaphragm 60 and the plate member 56, is a tube 54 whichprojects into the chamber 53 of the magnet thereby providing apassageway 64 connecting the interior of said chamber with theatmosphere adjacent the front surface of the diaphragm.

The operation of this device as a sound translating instrument forchanging acoustic vibrations to electrical vibrations is as follows: v

' The acoustic vibrations are impressed upon the front -surface of thediaphragm such that a portion thereof acts thereagainst and anotherportion passes through the tube 54 into the chamber 53 and acts upon therear surface of the diaphragm. Those vibrations however whichA act uponthe rear surface o the diaphragm are so changed in phase and magnitudethat they act to reenforce the effect of the vibrations impressed on thefront surface ofthe diaphragm whereby the amplitude of vibration of thelatter is increased.` .This reenforcing action is particularly prominentat low frequencies, a great increase in the response of the device overits response when not incorporatin this feature of the invention beingobtaine It will be understood, of course, that when acoustic vibrationsare impressed upon the diaphragm currents varying in accordance withsaid acoustic vibrations will be produced in the coil 61 because of itsmovement in the ield' provided by the magnet 50.

It will be apparent that the method out- A lined in the previousparagraph `for increasing the response of the transmitter at the lowfrequencies is similar tothat for increasing the response of a loudspeaking receiver such as that described with reference to Figs. 1 and2. It is believed, however, that a clearer understanding of thisinvention as applied to a transmitter of the moving coil type will beobtained from the consideration which follows ofthe factors involved inits operation.

The voltage generated by a moving coil transmitter is proportional tothe veloclty of the diaphragm. This fact makes the problem of getting auniform response down to low fre uencies diicult in a transmitter ofpractica sensitivity. It means that either the diaphragm lmust have avery low stiffness or 'some wa must be provided to amplify the action othe sound wa'ves on the diaphragm relative to that at higherfrequencies. f The method here described increases the force on thediaphragm at low frequencies sufficiently to compensate for thereactance of the edge stiffness down to relatively low fre uencies. Itis possible by this method to malntain a uniform response two octavesbelow the frequency at which the edge stiil'ness of the diaphragm beginsto show a decrease in the response characteristic of the transmitter.

'The force on the diaphragm at low frequencies is increased by means ofthe connecting tube 54 between the front of the dialli) lili

phragm 60 and the ehemher 53 beck ore the diaphragm whereby the pressureet the front of the diaphragm' ufterv heilig chenged both in magnitudeand phase is impressed on the hack of the diephragm. The magnitude endphase or the pressure on the'huck of the diuphragm ere controlled hy theecoustic constants of the air in the tube and chamber. Fig. 9 shows thetransmitter with the open end oi the connecting tube close to the die.-phragm so thet the phase of the pressure is `the same ever the diaphregmund tube opening. Fig. i@ is u circuit dlagrem with pressure applied tothe mechanical end ecoustic constants et the trensmitter. Fig. Ill is acircuit diagram or the trensmitter shown in Fig. 9 without the tube54.-. A. comparison p J,

er these twe .figures readily shows the change wrought therein hy theinsertion oi the tube. rlhe smell air chamber `between the diaphragm andecoustic resistance, R0, has been omitted in the diegrerns of Figs. l()and ll heceuse of its negligible eect et .low frequencies. Forconvenience the pressure end the constants of the transmitter have `beenreferred te the eective urea. ef the diaphragm; for examplea the forcellis equei te the open eir pressure at the diaphragm multiplied by theeiective eres. or the diaphragm.

Fiilie response or veituge generated by the moving coil of thetransmitter is proportlonel te the velocity the diaphragm dr vlded bythe force on the diaphragm. Es'.- pressing this retire in terms of themechenicel and ecoustlc constents of the transmitter for large tubes canbe used. The tube im A edunee coecient was therefore calculated rom thegeneral expression 600 ohms at about 50 cycles. llt will be suicientlyaccurate to use these average velues of impedence in the responseEquation (3).

The measured end calculated constants of etrensrnitter constructed inaccordance with this invention were Substituting these values inEqnetion (3) end solving for absolute velues gives the upper full linecurve A., shown in Fig. l2, plotted in decibels.

lf the tub-e 54s is closed Equation (3) rethere is obtained: duces to7Jc' v R lf- Q S l S Sit SR /3 L L1,

s.m+sm.s2m.m+sm 33 *new (nems---wmul Y is c: e: o;

in which v'" l T ,ma :erlective muse of.' the diaphragm, mass of the l R+37 wm gf- (4:)

coil and the mass of air in the resistance gap a 9 reexgfd totlegaggirahgm. T h =edge s ness o e .ap ragni. Jj" =resistance of theair gap' referre to the da# -n i t. e abgve Vaules are ubstluted DOW Hlphrsgm. equation (ai) the lower rull llne curve B., S :stiffness of theair in the transmitter chamber referre to the diaphragm. B. andm=impedance of the sir in the compensating tube referred te 'thediaphragm.

ln calculating the impedance eithe in the tuhe it was found that theradius (ro) of the tube gave e, veine er 'the discriminent fm, 112m/lL-.pw

between l and l in which neither Posseuilles coecient for narrow tullenor the simplified expression shown in'lig. 12, is obtained. |lihesecalculated response curves are indlcatlons of the greuter response atthe lower frequency to he expected of a given device when the methodbodied therein.

rlllie dotted curves C and D show the eX- perimentally measured responseof the transmitter in decibels when the tube is closed and openedrespectively. These measurements it will be noted e ree closely with thetheoretic-al calculations; t e variation in the two sets of curves below50 cycles can be accounted for by e, prohable error of 10% in themeasurement of the edge sti'ness of the diaphragm S0.

Fig. 13 shows the overall frequency respense of av moving coiltransmitter and assov ciated transformer between and 10,000 with thetube 54 opened and closed. It will end means' comprising this inventionare em be noted that above 200 cycles the responseof the particulartransmitter considered above appears to be unaffected by the presence ofthe passage-way between the front and rear of the diaphragm but thatbelow that frequency the increase in response is very marked.

It will be understood, of course, that althou h specific dimensions andproportions have een given in describing the application of thisinvention to sound translation devices, the embodiments` described areillustrative only of this invention which is to be considered as limitedonly by the scope of the appended claims.. v Y

What is claimed is: n i

1. A sound translating device comprisin a diaphragm, means definin anacoustic sti ness adjacent one surface o said diaphragm,

the other surface of said diaphragm being open to the atmosphere, andmeans defining an acoustic mass providing an acoustic path between thetwo surfaces of said diaphragm, said acoustic mass and stiffnessproportioned so that over a portion of the fre uency range the soundvibrations translated y the o posite surfaces of said diaphragm have teir magnitude and phase so related that they act to effectivelyreenforce each other whereby the amplitude of the translated vibrations'is increased.

2. A sound translating device comprising a diaphragm, one surface ofwhich is open to the atmosphere, means defining a Huid chamber enclosingthe other surface of said diaphragm, and means closely adjacent theperiphery of said dia hragm and defining a fluid passage-way lea ing romsaid chamber to the atmosphere and through which a movement of fluid mayoccur when said diaphragm is actuated whereby over a portion of thefrequency range the vibrations translated by each surface thereofreenforce each other thereby increasing the amplitude of the translatedvibration.

3. A soundtranslating device comprising a diaphragm one surface of whichis open to the atmosphere, means defining a fluid chamber enclosing theother surface of said diaphragm, and means defining a fluid passage-Away leading from said chamber to the atmosphere and through which amovement of fiuid into and out of said chamber may take place, the Huidin said chamber and said passageway having such stiffness and mass,respectively, that when the diaphragm is actuated the low frequencyvibrations translated by each surface are caused to have such magnitudeand phase relationship that they effectively reenfoi'ce each other toincrease the amplitude ofthe translated sound.

4. The method of increasing `the response of a loudspeaking device atthe llow frequencies which comprises utilizing the radiation from eachsurface of the diaphragm whereby the radiation resistance per unit areaofthediaphragm is e'ectively increased and caus`ng the sound waves fromone surface of the diaphragm to act in phase with those emanating lfromthe other surface by providing an acoustic 'stiffness and. mass throughwhich the sound emanations from the one surface must act beforecombining with those from the other surface. y i 5. A loudspeakingdevice comprising a daphragm, one surface of which is o en tothe atmoshere, means defining a flui chamber enclosing the other surface of saiddiaphragm, and means defining a fluid passageway leading from -saidchamber to the atmospliere closely adjacent the surface of saiddiaphragm open to the atmosphere, sad passage-way having an areasubstantially e ual to that of the diaphragm and when said iaphragm isactuated enabling fluid to be pumped into and out of said chamber whereyover a portion of the frequency range the acoustic disturbancesemanating from the two surfaces of said diaphragm combfne to reenforceeach other.

6. A loudspeaking receiver comprisin a diaphragm, means for driving saidiaphragm, and an enclosure for said driving means and the rear surfaceof said dia-cy phragm, said enclosure having an aperture therein ofsubstantially the same area as the diaphragm and adjacent thereto, thechamber behind said diaphragm and the volume enclosed by said apertureproviding the stiff- 'ness and `mass elements respectively of anacoustic filter through which sound disturbances over a portion of thefrequency range emanating from the rear surface of said diaphragin passand are shifted through approximately 180o before combining with thesound disturbances emanating from the front of the diaphragm.

7. A sound translating `device comprising a diaphragm, means forutilizing the front and rear surfaces of said diaphragm as effectivesound translating surfaces, said means comprising a fluid enclosure forthe rear surface of said diaphragm defining an acoustic stiffness, and atubular member leading into said fiuid enclosure connecting the sainewith the outside atmosphere adjacent the front surface of the diaphragmandv definin an acoustic mass, said acoustic mass and sti ness acting aselements in an acoustic filter to change the magnitude and phase of theacoustic "disturbances over a portion of the frequency range which aretransmitted to the rear surface of the diaphragm whereby saidd"sturbanccs` act to reenforce tlieeffect of the sound disturbancesimpressed on the front surface of said diaphragm to increase theamplitude of vibration thereof.

8. A sound translating device comprising a hollow magnet structuredening a magnetic air-gap, a diaphragm, means for supporting saiddiaphragm, driving means atbetween the atmosphere and said hollowedportion, the duid in the latter and that enclosed by said-tube havingsuch stiffness and mass, respectively, that the front surface issubjected to substantially all of the impressed acoustic disturbancesand the rear surface of said diaphragm is subjected to a portion only ofthe impressed acoustic disturbances, said mass and stidness acting tochange the magnitude and phase of a portion of said sound disturbancesbefore the latter act upon the rear surface of said diaphragm wherebythe disturbances impressed on opposite' sides of said diaphragmreenforce one auf other to increase the amplitude of vibration of thediaphragm.

9. A sound translating device comprising a sound translating diaphragmhaving a driving means secured thereto and an enclosure for one surfaceof said diaphragm, theother surface being open to the atmosphere, said'enclosure .providing a shunt stiffness into which said one surface ofsaid diaphragm works, and means providing an acoustic passage from saidenclosure to the atmosphere adjacent the other surface of saiddiaphragm,

the area of said acoustic passage being substantially equal to theedective area of the diaphragm and the mass of the fluid therein beingsubstantially equal to the ed'ective mass of the diaphragm andsaiddriving means.

10. A sound translating device of the loudspeaking receiver typecomprising a sound translating diaphragm having a drivingl means securedthereto, an enclosure for one surface of said diaphragm defining a bodyof uid there adjacent, said fluid providing an acoustic stidness intowhich said one surface of said diaphragm works, and means providing an.acoustic passage between the enclosed surface and the other surface ofsaid diaphragm, the duid in said passage having a mass substantiallyequal to the edective mass of said diaphragm and said driving means andthe duid in said enclosure an acoustic stidness such that the lowfrequency sounds translated by the enclosed surface of said diaphragmact through said stiffness and mass to reenforce the sounds translatedby' the other surface of said diaphragm.

ll. A sound translating device comprising a diaphragm and means definingan acoustic filter of the lowpass type between the two surfaces of saiddiaphragm, the stiffness and mass element of said 4filter proportionedwith reference to said diaphragm so that sound vibrations translated bythe two surfaces thereof are eectively utilized to increase the responseof said device at the low frequenciesl .over that of a device withoutsaid means.

-ilter of the low pass type providing an acoustic path between the twosurfaces of said diaphragm, said filter so proportioned that the lowfrequency sounds translated by said surface of each diaphragm are sorelated in phase and magnitude as to e'ectively reenforce each other.

13. A sound translating device comprising a diaphragm and means defininganacoustic filter of the low pass type providing an acoustic pathbetween the two surfaces of said diaphragm, the mass and stid'ncsselements of said filter proportioned so that the phase of the lowfrequency sounds translated by the surface of said diaphragm issubstantially reversed with reference to that of the sounds translatedvby the other surface of the diaphragm whereby the response of saiddevice is increased over that of a device without said means.`

le. A sound translating device comprising a diaphragm and means deningan acoustic filter providing an acoustic path between the two surfacesof said diaphragm whereby the sounds translated by one surface of saiddiaphragm are caused to so act with reference to those translated by theother surface of said diaphragm so that-the response of thel device isincreased at the lower .frequencies over that of a device not havingsaid means, said filter acting to substantially reverse the soundvibrations propagated therethrough and translated by said one surface.

15. A sound translating device comprising a diaphragm and means definingan acoustic filter providing an acoustic path between the oppositesurfaces of said diaphragm, the stiffness 'and mass of said lterproportioned so that the cutod frequency is in the region of the lowestfrequency to be translated.

i6. A sound translating device comprising a diaphragm and means definingan acoustic lter providing anacoustic path between the opposite surfacesthereof, the sti'ness and mass elements of said filter proportioned sothat the acoustic vibrations translated by each surface of saiddiaphragm are caused to reenforce each other at low frequency.

17. In an acoustic device, a diaphragm;

means substantially closing one surface of said diaphragm from theatmosphere and defining an air chamber there-adjacent, and means foracoustically coupling the opposite surfaces of said diaphragm, saidmeans comprising a tubular member having one end open. to the atmosphereand the other end within and opening into said air chamber.

18. In an acoustic device, a diaphragm, means substantially closing onesurface of said diaphragm from the atmosphere and delining an airchamber there-adjacent, and

genaue f means for coupling the opposite surfaces of said diaphragmthrough said air chamber ,so that sound waves originating on the exposedVsurface of the diaphragm act on opposite surfaces of the diaphragm toeiectively increase the response thereof over the response with soundwaves acting on one surface only, said means comprising a tubular memberhavin an open end in the vicinity of the expose surface of said diaphraand another open end within the air chamildi. i

19. An acoustic device. comprising a diaphragm, drivingY means attachedthereto, an ,enclosure for one surface of said diaphragm, the othersurface being exposed to the atmospiere, said enclosure providing an airchamr into which said one surface of the diaphragm works, and meansproviding an acoustic passage from said'enclosure to the atmosphere, thelarea of said, passage being substantially equal to the effective areaof the diaphragm and the mass of the fluid therein being substantiallyequal to the effective mass of the diaphragm and driving means, saidmeans comprismg` aplurality of tubular members.

20. Anacoustic `device comprising a diaphragm, driving means attachedthereto, an enclosure for one surface of said diaphragm, the othersurface being exposed to the atmosphere, said enclosure providing an airchamer into which said one surface of the diaphragm works, and meansproviding an acoustic passage -from said enclosure to the atmosphere,the `area of said assage being substantially e ual to the effective areaof the diaphra an the mass of the uid therein bein su stantially equalto the eil'ective mass of t e diaphragm and driving means, said meanscomprising a plurality of concentric spaced anuli.

21. Anacoustic device comprising a diaphragm, driving means attachedthereto, an

the .diaphragm and the mass of "the fluid enclosure for one surface ofsaid diaphragm, the other surface being exposed to the atmoshere, saidenclosure providing an air cham- Ber into which said one surface of thedia- .phragmworks, and means providing an acoustic` passage from .saidenclosure to the atmosphere, the area of said passage bein substantiallyequal to the effective area ol therein being substantially equal to theeiective mass, of the diaphragm and driving means, said means comprisinga plurality of individual tubes each defining of the acoustic passage. y

22. An acoustic device comprising a diaphragm, an air chamber onthe'back side of the diaphragm, a second and larger air chamber on theback side of the diaphragm, said chambers being connected by a narrowpassageway, and hollow'means providinga passage for sound waves of aportion of the nu audio frequency range between the front .impedance towaves of a frequency a portionchamber.

23. An acoustic device comprising a casing, said casing comprising aplurality of wall members deiining an air chamber, one

of said wall members having an opening 'side o`f the diaphragm and saidlarger air 24. A n acoustic device comprising a da l phragm, an airchamber on the back side of the diaphragm, a second and larger airchamber on vthe back side of the diaphragm,

chambers being connected by a narrow passageway, and means providingapassage for sound waves from the front side of the diaphragm to saidlarger air chamber, said passage having an 'impedance characteristicsuch that substantially no waves having a frequency `greater thanapproximately two hundred cycles per second pass therethro h. .25. Anacoustic device comprisin a aphragm, an enclosure providing an aircha'rnber on the back side of said diaphragm, and means providing a.passage from the front side of said diaphragmto said air chamber, saidmeans comprising a hollow member o en at its ends, said passage oiferingsuch a lugh eater than approximately two hundred cyc es r second thatsubstantially no waves above t at frequency pass therethrough.

26. An acoustic device comprising a diaphragm, aV casing, said casingproviding a substantially closed air chamber on one side of saiddiaphragm, said casing having an4 and a tubular member open at each enda cent said casing and providing a continuation of said opening, thecasing Vchamber and said opening therein. adjacent said diaphragi,

tubular member being proportioned so that -sound waves in -the regionbelow approxiing an air chamber on the other surface of thediaphragm,and means for fixing the lower cut-off of the device, said meanscomprising a hollow air enclosing member open at each end, extendinginto said chamber, and connecting the latter with the atmosphereadjacent `yIthe exposed surface of the diaphragm. Y

In witness whereofl I hereunto subscribe my name this 13th day ofAugust, 1930.

ALBERT L. THURAS.

